The present invention relates to the field of wellbores drilled through subsurface geologic formations. More particularly, the invention relates to controlled breakouts for enhancing borehole stability.
Wellbore stability significantly impacts the efficient drilling and production from hydrocarbon producing wells from subsurface geologic formations. As vertical and horizontal wellbores are drilled deeper and farther into geologic formations, integrated rock mechanics analysis assesses the risk of wellbore instability.
Wellbore "breakouts" are defined as the partial failure of the wellbore wall due to high stress concentrations. Breakout of the wellbore wall is conventionally considered to comprise wellbore failure because such breakout leads to wellbore blockage and uncontrolled wellbore enlargement. Conventional drilling practice seeks to minimize the maximum value of stress concentration on the borehole wall. This is accomplished by optimizing drilling trajectory and by circulating a heavy drilling mud through the wellbore. However, heavy mud weights can introduce hydraulic fracture and result in circulation losses through the formation, and can cause near wellbore impairment of the hydrocarbon producing formation.
Breakouts occur from a series of failures on the wellbore wall as stress concentration exceeds formation strength at that location. Known elastic equations model the stress distribution around a wellbore in an arbitrary stress field, where .sigma. and .tau. having subscripts r and .theta. represent the effective normal and shear stresses in a cylindrical coordinate system with z axis parallel to the drilling direction; .sigma. and .tau. having subscripts xx and xy represent the effective normal and shear stresses with a Cartesian coordinate system having the same z axis as the cylindrical system; r is the radial distance from the center of the wellbore and a is the borehole radius; and .theta. is the azimuthal angle measured from the x axis. Under elastic conditions, the maximum stress concentration occurs on the wellbore wall where r=a. The principal stresses at a location on the wellbore wall are described as: ##EQU1##
where .sigma..sub.tmax and .sigma..sub.tmin are the maximum and minimum effective principal stresses on the tangential plane of the wellbore wall. Failure occurs when the maximum principle stress exceeds the effective strength, as represented by: EQU .sigma..sub.tmax.gtoreq.UCS+.sigma..sub.3 tan.sup.2 (.pi./4+.phi./2) (2)
wherein UCS is the unconfined compressive strength and .phi. is the internal friction angle of the rock.
Conventional practice for drilling a mechanically stable wellbore concentrates on reducing the maximum tangential stress on the wellbore wall to below the effective strength described as the converse of Equation (2). In-situ stress orientations, magnitudes, and rock strengths comprise parameters that cannot be controlled. Consequently, the factors conventionally controlled by an operator are drilling direction and the mud weight. For example, under normal stress conditions, when the vertical stress is the maximum stress, conventional drilling practice advocates for horizontal wells parallel to the minimum horizontal stress. This path is also identified as the direction yielding the lowest value of maximum tangential stress on the wellbore wall.
Conventional drilling practice seeks to avoid breakouts completely by reducing the maximum wellbore wall principal stresses. This is generally accomplished by increasing mud pressure until the value of the maximum wellbore wall stress is less than the strength of the formation. When drilling highly inclined or horizontal wells, effort is made to orient the wellbore in a direction to reduce the maximum wall stress.
Although this approach can generate a wellbore without breakout by minimizing the maximum value of tangential stress, this approach does not provide stability during production of hydrocarbon fluids. During open hole production, the bottom hole pressure is equal to or less than the near wellbore pore pressure, thereby removing the stabilizing effect of weighted drilling mud. The increase of mud pressure or drilling parallel to minimize horizontal stress (to minimize the maximum stress concentration) prevents breakout during drilling operations, however the removal of excess mud pressure during open hole production leads to breakout.
Accordingly, a need exists for new approach for drilling and maintaining the stability of a wellbore drilled through subsurface geologic formations.